Suppose, there are 100 images on a tape (which shows up as FULL) , and out of that 5 are valid (read available) ,as is your case; this does not mean that the rest 95 are invalid The terminology for this is called expired images.
Now, I'm not sure how other backup softwares treat the tapes in this condition, but
netbackup will consider a tape as available when all the images on the tape have expired. Until then, Netbackup will not rewind the tape and write backups in the 'empty' space.
Netbackup will utilize the tape (mark that tape as available) only when all the images on the tape have expired, and there is no way (that I am aware of) to force netbackup to write backups from the beginning of the tape if the tape contains unexpired images in the end
This is why you would see that a tape is mounted and '
positioned to file # ' before bptm starts writing data.
Makes Sense?